The present invention relates to a drum brake device in which a pushing force of the brake shoe against the brake drum is controlled according to a reaction force so that the brake can be highly effectively applied and stability of the brake can be maintained. More particularly, the present invention relates to improvements in the drum brake device for smoothly transmitting an input of brake at the time of applying the brake.
In order to brake a running vehicle, various types of drum brakes have been conventionally used. These drum brake devices are classified into a leading trailing type drum brake, a two leading type drum brake and a duo-servo type drum brake by the arrangement of brake shoes which are pushed onto the inner circumferential face of the substantially cylindrical drum.
In general, the duo-servo type drum brake device has a pair of brake shoes including a primary shoe and secondary shoe arranged in a cylindrical drum being opposed to each other.
In the primary shoe, the entry side in the forward rotating direction of the drum is an input portion, and the exit side in the forward rotating direction of the drum is connected to an entry side of the secondary shoe, for example, via an adjuster. On the other hand, the exit side of the secondary shoe is made to come into contact with the anchor portion provided on the backing plate, and a reaction force (braking torque) given to the primary and the secondary shoes are received by the anchor portion.
Due to the foregoing, when the primary and the secondary shoe are expanded and pushed onto an inner circumferential face of the brake drum, a reaction force given to the primary shoe is inputted onto the entry side of the secondary shoe, so that the secondary shoe can be pushed onto the inner circumferential face of the brake drum. Therefore, a self-servo action is given to both the primary and the secondary shoe. Accordingly, it is possible to obtain a braking force of a very high gain.
Compared with a leading trailing type and a two leading type drum brake device, the aforementioned duo-servo brake device has the following advantages. It is possible for the duo-servo brake device to provide a very high intensity of braking force. Further, the duo-servo brake device can be easily downsized. Furthermore, a parking brake can be easily incorporated into the duo-servo brake device.
However, the above duo-servo brake device is sensitive to a change in the coefficient of friction of the brake shoe lining. Accordingly, there is a tendency that the braking force is difficult to be stabilized. Therefore, it is necessary to devise to stabilize an intensity of the braking force.
Concerning the recent brake device for vehicle use, in order to provide a brake system having a highly intelligent braking function such as an anti-lock brake system, or in order to cope with an electric vehicle (EV vehicle) which is appropriate to reduce environmental pollution, developing of the electric brake device is a very important task to be accomplished.
According to the background described above, the present applicant has already proposed the following link mechanism to be used as a shoe drive mechanism for expanding brake shoes at the time of applying the brake. At the time of service braking operation, a pair of brake shoes are expanded and pushed against a drum according to a shoe operating force transmitted from the operating force generating means to the input lever. On the other hand, when an intensity of the reaction force given to the anchor pin is increased by a predetermined magnification with respect to an intensity of the shoe operating force, a brake restricting force, which acts in a direction so that an action of the shoe operating force can be reduced, is given to the input lever by the link mechanism. (For example, refer to JP-A-2001-254766.)
When this link mechanism is used, it is possible to stabilize a braking force of the duo-servo type drum brake device. Further, when an electric operating force generating means, into which an electric motor and others are incorporated, is employed as the above operating force generating means instead of a conventional hydraulic wheel cylinder, the electric brake device can be easily realized.
However, in the case of the above conventional link mechanism to be used as a shoe drive mechanism, the following problems may be encountered. In the aforementioned shoe drive mechanism, a reaction force at the time of applying the brake is fed back to an input lever so as to control the braking force. Therefore, the reaction force fed back to the input lever at the time of applying the brake obstructs an input transmission into the brake shoe. Therefore, the input transmission can not be smoothly conducted, and the input transmission efficiency is deteriorated.